It looks like they put bogus date codes in the gzip header, which caused them to be flagged as invalid. With the recent updates to binwalk, this level of date validation is no longer necessary to prevent false positives anyway, so the latest code in the master branch no longer marks them as invalid (it does indicate to the end user that the dates appear bogus though).

It uses an AMRISC microcontroller, whose architecture/instruction set I am not familiar with, so I can’t tell you much about the firmware. What I can say about it is:

1) This is not running Linux
2) There are sections of the firmware that appear to be code
3) There are sections of the firmware that appear to be compressed, but not with a compression format that I’m familiar with

To analyze this firmware, you’d first want an AMRSIC disassembler. Then you could reverse the code to figure out the compression, decompress it, and further reverse the firmware.

I’d expect so; assuming that there is compression (and I’m pretty sure there is), then something has do the decompression at some point. Whether this code is included in the firmware update image or not, I don’t know (might be in the bootloader, which is not always included in the firmware update file).

Doing stuff like this is hard on file with mixed files.
So…I recommend to add option to binwalk to cut-out unnecessary files(jpg,gif, etc) and on situation when we example have a.gif+unnkowfile+b.gif+unnknowfile2 it’s recommended to add something that will separate unnkowfile and unnkowfile2 or just split them.
GIF files end’s with 00 3B (binwalk is extracting them incorrectly(goes too far))
JPG starts with FF D8 and ends with FF D9
It’s harder with htm and js files but with access to router it’s possible to get every html and js file and get know when they starts and ends

Are you interested in trying to extract the firmware of DIR-850L (hardware version B)? Its firmware is rather strange that its file structure does not look like the firmware of other models. binwalk does not return any result. The entropy is consistent throughout the whole file. The entropy is so high that I am quite sure that it is compressed (while for other normal D-Link firmwares, there are some parts of the file not compressed and results in a low entropy).

The firmware of DIR-850L (hardware version A) is normal. I have also tried many other models of D-Link and their firmwares are normal as well.